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ISSUE 13  JULY 29, 1999

 

SUNFLOWER MIDGE UPDATE

    Reports and field inspections from throughout the eastern part of the state indicate sunflower midge larvae are present in buds. The flat, oval, creamy-white larvae can be found at the base of the bracts. The scarring caused by their feeding can be found with the larvae.

    There have been several reports of significant midge numbers in buds from the Starkweather-Webster areas of Ramsey County. Last years area of greatest concern was from an area around Hope to Cooperstown and south to Sanborn.

    It is still too early to assess the level of damage these infestations may inflict on the plants. Variables that influence the level of injury include the number of larvae present, bud size at the time of infestation, and sunflower hybrid being grown. Last year, fields that displayed damage symptoms still yielded respectably because tolerant hybrids were raised.

    Other impacts the midge has on plants is delayed flowering and destruction of ray petals. In fields where injury is concentrated on the ray petals, fields may not take on the normal yellow appearance we expect when flowering is underway. However, closer inspection often reveals that the florets in the face of the flower sustain less damage and seed are still produced in the flower. The important message is not to judge too hastily the potential losses when midge larvae are found.

    It is time to inspect flowers for the presence of larvae, determine if infestations are confined to field margins or throughout the field, and begin to assess extent of damage (scarring in bract areas only, feeding throughout the bud with floret damage, severe cupping of heads).

 

RED SUNFLOWER SEED WEEVIL AROUND THE CORNER

    Early reports of red seed weevil activity in the state, however, nothing significant. As we approach flowering in the earlier planted fields, it is time to plan scouting activities.

Treatment Threshold - Confection sunflower

    The economic threshold for red sunflower seed weevil on confection sunflower is based on the need to keep seed damage below the 3 to 4 percent industry standard. Assuming confection sunflower contains 800 seeds per head, the number of damaged seeds per head would therefore need to be kept below 24 to 32, to remain below the industry standard of 3 to 4 percent seed damage. Research on oilseed sunflower indicates that for each weevil sampled in the early bloom stage, 27 damaged seeds resulted. This suggests about one weevil per head as an economic threshold for red sunflower seed weevil on confection sunflower.

Calculating Economic Thresholds for the Red Sunflower Seed Weevil-Oilseed sunflower

    To decide whether to use an insecticide treatment to control red sunflower seed weevils, it is necessary to determine the economic threshold for this year. The economic threshold (ET) for red sunflower seed weevil depends on the following variables:

    a. the cost of insecticide treatment per acre;
    b. the market price of sunflower in dollars per pound;
    c. the plant population per acre.

ET =                       Cost of Insecticide Treatment                           
             Market Price x 21.5 ((0.000022 x Plant Pop'n) + 0.18)

 

Red Seed Weevil Economic Thresholds (Weevils per head)

Plant population = 18,000 per acre

Market
Price ($)

Treatment Cost ($/A)

6.00

7.00

8.00

9.00

10.00

11.00

0.07

7

8

9

10

12

13

0.08

6

7

8

9

10

11

0.09

5

6

7

8

9

10

0.10

5

6

6

7

8

9

0.11

4

5

6

7

7

8

0.12

4

5

5

6

7

7

0.13

4

4

5

6

6

7

Price for Oilseed Sunflowers = $0.08

Plant
Population

Treatment Cost ($)

6.00

7.00

8.00

9.00

10.00

11.00

17,000

6

7

8

9

10

12

18,000

6

7

8

9

10

12

19,000

6

7

8

9

10

11

20,000

6

7

8

8

9

11

21,000

5

6

7

8

9

10

22,000

5

6

7

8

9

10

23,000

5

6

7

8

8

9

24,000

5

6

7

7

8

9

25,000

5

6

6

7

8

9


Timing treatments

    Sunflower plant stage is used to time insecticide treatment. The ideal plant stage to treat is when most plants in the field are at 40 percent pollen shed. However, we recommend that treatment be considered when more than half of the plants in the field are just beginning to show yellow ray petals to 30 percent of the heads shedding pollen and the rest of the plants in the field are still in the bud stage. This difference between the ideal plant stage (40 percent pollen shed) to treat and the earlier plant stage (just beginning pollen shed) is based, in part, on the fact that aerial applicators - because of a busy schedule or adverse weather - will not always be available to spray at the ideal stage of sunflower development. The consideration of treatment at the early bloom stage should allow growers a sufficient cushion of time to have their fields treated. Growers must be aware, however, that if weevil populations are high and/or spraying is done too early, a reinfestation may occur and a second insecticide application may be necessary.

    Although insecticides applied to sunflower at the bud stage will kill weevils, treatments at that stage are not economical or effective because (1) seeds have not developed to a stage suitable for oviposition, (2) eggs within the weevil are not mature, and (3) adult weevil emergence is still continuing. Sunflower normally reaches the bud stage in late July at which time only about 30 percent of the weevils in the soil have pupated and emerged. Most weevils emerge from the soil by the first week of August. If growers were to spray bud stage sunflower in mid to late July, a second spray may be necessary as more weevils continue to emerge.

 CORN BORER UPDATE

    Many treatment decisions are being made this week around the state. In general, populations are much smaller than we have dealt with the previous five seasons. There was a flush of moth activity detected in black light traps in Dickinson and Minot, late last week. Watch these western areas for egg masses and larvae this week to determine if threshold levels will be reached. In the southeast, treatment is underway in field corn, but very few fields have threshold levels of borers present. Not every field has treatable numbers of larvae, but decisions should be possible this week.

Phillip Glogoza
Extension Entomologist


APHID ALERT - UPDATE 23 JULY

   visit Aphid Alert on the WWW:

    http:/ipmworld.umn.edu/alert.htm

    During the week ending 20 July, green peach aphid captures remained at essentially the same level per trap as in the previous week. We anticipate green peach captures in the traps will remain relative stable until they start moving from potatoes. This begins in early August as the potato crop matures and aphid numbers on commercial potato production increases. Captures of PVY vectors (counts include potato aphid, but not green peach aphid) increased about 45% over the previous week. This increase in PVY vectors probably reflects the maturation of small grains and canola causing the insects to seek alternative hosts.

    It was reported on July 26 that all winged green peach aphids trapped last week have tested negative for both PVY and PLRV. No positives have been reported yet this year.

Source: Dr. Ted Radcliffe
University of Minnesota


EVALUATE WHEAT MIDGE LARVAL INFESTATIONS

    Midge larvae are present in wheat heads, now. It is time to evaluate midge levels in those heads. An infested wheat head does not change its appearance so it is necessary remove the glume and expose the kernel or rub drier heads. We would suggest that random samples of wheat heads (approx. 20 heads per site, 4 to 5 locations) be used for this type of evaluation. This needs to be determined before midge drop from the heads after rainfall or heavy dews.

    To evaluate a field for potential yield reduction, you want to establish the percent infestation level for the field, not just selecting some of the worst infested wheat heads.

    Research has shown that the percentage yield loss is closely related to the percentage of kernels infested with wheat midge. Entomologists estimate that an average of 13 larvae per head will reduce wheat yields to a point where control becomes profitable. For example, an infestation level of 2% reflects a larval population of 1-2 larvae per head (or 0.033 larva per kernel). The relationship between yield loss and the number of midge larvae and infestation level is summarized in the table
below.

Larvae per kernel

% infested kernels

Estimated % Yield Loss

1

30-38%

40%

2

58-60%

65%

3

78%

---

4

90-96%

79%

Janet J. Knodel
Area Extension Specialist Crop Protection


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